After a paperweb is formed it typically undergoes further treatment to modify its properties. One typical process is to calender the paper web. The paperweb is passed through a calender which typically comprises a series of nips formed between one or more pairs of rolls. The calender conventionally smoothes the surface of the paperweb. During the calendaring process, the thickness or caliper of the paper web is reduced and the paperweb is densified. The density of the resulting paperweb is typically calculated as:Density=Basis Weight/Caliperwhere the basis weight is the weight of a ream of the paperweb, in pounds, and the caliper is the thickness of the paperweb, measured in thousandths of an inch, or points. Since calendering generally reduces caliper, paper that is calendered has a higher density than uncalendered paper. The bulk of the paper is inversely related to density, therefore when the density is increased, the bulk of the paper will be reduced.
Calendering is typically performed using a gloss calender, soft calender or supercalender. The gloss calender is typically comprised of a hard, non-resilient, heated roll made, for example, of steel, positioned proximally to a soft roll so as to form a narrow gap or nip. As the paperweb passes through the calender nips it is exposed to a nip load in the range of from about 100 to about 900 pounds per lineal inch (pli). Nip pressures in gloss calenders are usually in the range of less than about 2000 pounds per square inch (psi). A wide range of processing temperatures can be used in a gloss calender, with the typical high temperature being in the range of about 450° F.
The finishing effect achieved using a gloss calender, however, is not as smooth or flat and therefore not as glossy as the surface produced using higher pressures on the paperweb. It is conventionally known to increase the nip load or the roll temperature, or both, to plasticize and smooth the surface layers of the paperweb. Such modifications are incorporated, for example, in the design and operation of a conventional soft calender. The soft calender is usually constructed as having one to two nips per coated side, or as a two or four-nip device, with each nip being formed between a heated hard roll and an unheated soft roll.
An alternative method is to use a supercalender wherein the paperweb is sequentially passed between a series of nips formed between vertically stacked rolls of a supercalender. The supercalender typically comprises a frame having an upper roll and a lower roll with intermediate rolls positioned in between. The rolls of the supercalender may be heated hard rolls or unheated soft rolls, in serial or alternating arrangement. The nips formed between the rolls are typically shorter than those of a soft calender or gloss calender. The upper temperature range of the heated rolls in the supercalender is usually about 250° F. As the paperweb is passed through each nip, the paperweb is compacted to form paper of substantially uniform density and high gloss. The high pressure of superclander however causes a reduction in the paperweb's bulk. In a supercalender, the nips are loaded initially by gravity, i.e., gravitational forces acting on the weight of the rolls produces weight distribution from the upper nip to the bottom nip that is substantially linear and increasing. This has the consequence that the load present in the bottom nip actually determines the minimum loading capacity of the calender.
Paper grades are often sold by surface area; thus a lower density sheet provides more surface area per ton of paper. This arrangement is often advantageous for both the manufacturer and the buyer. Thus it will be appreciated that a manufacturing method that provide a desired surface finish on the paperweb without substantially affecting its bulk is desirable. Conventional supercalender have difficulty maintaining more bulk in the paperweb because such a process requires relatively high initial nip loads and corresponding nip pressures, which often increase as the paperweb moves through the calender. A typical 10–12 roll supercalender device will produce a minimum load on the bottom nip in excess of about 1000 pli which could translate to a nip pressure greater than about 2500 psi depending upon the nip width. Furthermore, to achieve some calendering potential from the upper nips, additional external load must be applied to the rolls. For example, where the initial nip load may be about 1000 pounds per linear inch (pli) as the paperweb enters the first nip, it is then exposed to subsequent nip loads at each of the successive intervening nips before passing through a final nip at a cumulative nip load of about 2000–3000 pli. This amount of pressurization in combination with heat results in a paperweb that is highly densified with a high gloss surface. While the paperweb has a good finish it results in increase web density and loss of paperweb bulk. A comparison between supercalendering and gloss calendering is reported in the article entitled “Supercalendering and Soft Nip Calendering Compared”, by John D. Peel, TAPPI Journal, October 1991, pp. 179–186.
A recent development in the calendering art addresses the problem of increasing linear loads at the successive nips in a supercalender. U.S. Pat. No. 5,438,920 describes a modified calender that is comprised of a series of rolls similar to a conventional supercalender. However the loading at each nip can be controlled by way of relief means that partially or completely relieve the nip loads produced by the masses of the intermediate rolls. As the paperweb passes through this calender, there is less variation in the nip load and nip pressure that is applied at each nip. As a result, there is less reduction in the bulk of the finished paper. This patent does not, however, teach or suggest making a high gloss paper of reduced bulk. Laid-open Canadian Patent Application 2238466AA, filed Dec. 20, 1998, teaches using another type of modified calender with reduced nip loads at each nip to make an ultra-light weight coated (ULWC) paper, which is a high-bulk glossed paper.
It is known in the papermaking art that various coating formulations and coating ingredients may be used in the manufacture of paper to achieve high gloss. For example, U.S. Pat. No. 5,283,129 discloses a lightweight paper stock that is coated with a pigment composition. U.S. Pat. No. 4,010,307 discloses a high gloss coated paper product comprising calcium carbonate and a non-film forming polymeric pigment. U.S. Pat. No. 5,360,657 discloses a high gloss paper with a thermoplastic polymeric latex applied to paper before calendaring. Laid-open Canadian Patent Application CA 2238466AA describes the manufacture of an ultra light weight (ULWC) paper by applying a plastic coating pigment onto a base paper containing 60% weight or more mechanical pulp. The coated paper is then calendered at a nip loading less than conventional supercalendering nip loading, to produce a product having a bulk factor above 51 if a supercalender is used, and a bulk factor above 60 if a hot-soft calender is used. The maximum TAPPI 75° gloss achieved for ULWC paper using the invention of CA 2238466AA was reported as 35, while the inventors reported producing lightweight coated paper of lesser bulk having a maximum gloss value of 45. PCT published application WO 98/20201 discloses a method of making paper having high brightness and gloss by applying a coating comprising at least 80 parts precipitated calcium carbonate and at least 5 parts of an acrylic styrene copolymer hollow sphere plastic pigment, based on 100 parts total weight of pigment, before finishing the coated paper to achieve gloss development. The finishing process does not involve using a modified supercalender, and the resulting paper is not a high bulk product. Hollow sphere pigments have also been used to produce a non-gloss finish. U.S. Pat. No. 5,902,453 teaches applying a coating containing 30–60% weight hollow sphere particle pigments and 40–70% weight cationic starch binder to a web, then calendering, under unspecified conditions, to yield a product with an uncoated appearance rather than a gloss finish. In an article entitled “Lightweight Coated Magazine Papers,” published in the Jul. 5, 1976 issue of the magazine PAPER, Vol. 186, No. 1, at pages 35–38, a relationship between calendering and the use of plastic pigments in coatings is disclosed. Other publications, including the articles entitled “Light Reflectance of Spherical Pigments in Paper Coatings,” by J. Borch and P. Lepoutre, published in TAPPI, February 1978, Vol. 61, No. 2, at pages 45–48; “Plastic Pigments in Paper Coatings,” by B. Aluice and P. Lepoutre, published in TAPPI, May 1980, Vol. 63, No. 5, at pages 49–53; “Hollow-Sphere Polymer Pigment in Paper Coating,” by J. E. Young, published in TAPPI, May 1985, Vol. 68, No. 5, at pages 102–105, all recognize the use of polymer pigments in paper coatings.
What is needed is a method of manufacturing using a supercalender that results in a high bulk paper with a high gloss surface.